In an optical apparatus, the handling of unnecessary light inside the apparatus is important. Herein, unnecessary light is defined as light propagated along unintended light paths inside the optical apparatus and not used to achieve the intrinsic functions of the optical system. Such unnecessary light frequently causes degradation in the performance of the optical apparatus.
As a method for obtaining a large-screen image, for example, a projection image display device is known wherein an optical image corresponding to an image signal is formed on a light valve and the optical image is magnified and projected on a screen by a projection lens. As an example of this kind of projection image display device, a device using a reflective light valve is available wherein an optical image is formed by controlling the traveling direction of illumination light in accordance with an image signal. Such a projection image display device using a reflective light valve has high light utilization efficiency and can display projection images having high luminance.
In a projection display device using a reflective light valve, illumination light components not entering its projection lens become unnecessary light that is the so-called OFF light. However, if no countermeasures are taken, the OFF light is reflected by prisms disposed around the light valve, mechanical parts for holding various optical elements, etc. and eventually enters the projection lens. If the OFF light eventually enters the projection lens, the quality of an image to be displayed on the screen is degraded significantly. Hence, in this kind of projection display device, an absorbing plate coated with black paint has been used conventionally to absorb the OFF light (for example, refer to Japanese Laid-open Patent Publication No. 2001-66693).
In addition, as another example, a countermeasure for unnecessary light in the above-mentioned projection lens and the lens barrel for holding the image-taking optical system being used in optical apparatuses, such as digital still cameras and camcorders, has been known conventionally. Generally, the light reflected between the faces of the lenses inside a lens barrel and the light reflected by the mechanical parts for holding various optical elements become unnecessary light referred to as stray light. The stray light may occasionally return to the light path of the optical system along complicated reflection light paths. In the cases of the above-mentioned projection lens and an imaging optical system, such as the image-taking optical system of a digital still camera or the like, the stray light causes ghost or flare in the optical system, thereby causing degradation in the image quality of an image to be formed. Hence, in a conventional lens barrel, the internal face of the barrel is made of a black material or matte finished to prevent generation of stray light (for example, refer to Japanese Laid-open Patent Publication No. 2003-266580).